(1) Field of the Invention
The present invention relates to the field of optics and more particularly to the field of optics for controlling, focusing and imaging of neutron beams.
(2) Description of the Related Art
The optical properties of materials are characterized by their refractive indices. In the case of cold and thermal neutrons, the refractive index is slightly less than unity for most elements and their isotopes. See V. F. Sears, Neutron Optics, Oxford University Press, 1989, 64. Consequently, thermal and cold neutrons can be reflected from smooth surfaces at shallow ‘grazing-incidence’ angles (total external reflection) or be refracted at boundaries of different materials.
Optical elements for neutrons can be designed to concentrate the neutron current or to produce a true image of the neutron source. An example of the former is polycapillary optics in which neutrons undergo multiple reflections from capillary walls to emerge in a new direction. See D. F. R. Mildner, H. H. Chen-Mayer, W. M. Gibson, A. J. Schultz, Proc. SPIE 4785 (2002) 43. Arrays of capillaries, with a common focus, can converge a quasi-parallel beam of neutrons to increase the current density. See H. Chen, R. G. Downing, D. F. R. Mildner, W. M. Gibson, M. A. Kumakhov, I. Yu. Ponomarev, M. V. Gubarev, Nature 357 (1992) 391. Alternatively, concave refractive lenses [see T. Cremer, M. A. Piestrup, C. K. Gary, R. H. Pantell, C. J. Glinka, Appl. Phys. Lett. 85 (2004) 494] can be used both for neutron flux enhancement and for true imaging. However, the refractive index depends on the square of the neutron wavelength so that refractive optics are strongly chromatic and high performance can only be achieved with monochromatic neutron beams. Neutron optics based on total external reflection are achromatic, but to date these have been limited to toroidal single-bounce mirror systems [see C. Hayes, C. Lartigue, A. Kollmar, J. R. D. Copley, B. Alefeld, F. Mezei, D. Richter, T. Springer, J. Phys. Soc. Jpn. 65 (Suppl. A) (1996) 312] with higher aberrations than refractive lenses, or Kirkpatrick-Baez optics [see G. E. Ice, C. R. Hubbard, B. C. Larson, J. W. L. Pang, J. D. Budai, S. Spooner, S. V. Vogel, Nucl. Instr. and Meth. A 539 (2005) 312]. The latter feature two successive reflections in orthogonal directions but their usefulness is limited to small cross-section neutron beams if high imaging performance is required.
Reflective optics based on the so-called Wolter geometries [see H. Wolter, Annalen der Physik 445 (1952) 28] that are used extensively in X-ray astronomy because they minimize optical aberrations for off-axis rays, can also be designed for use with neutron beams. The optical scheme most widely used in x-ray astronomy is a Wolter-1 geometry whereby two consecutive reflections from parabolic and hyperbolic surfaces are used to focus the X-rays, as shown in FIG. 1. The mirrors are conical section of revolution (have a “cylindrical” form), so that optics with different diameters, but the same focal length, can be nested together to increase the system throughput. Since total external reflection optics requires a near parallel incident beam, they must be placed far enough from the neutron source to ensure a small incidence beam divergence.
Nested Wolter-1 geometry optics can greatly improve the focused neutron beam intensity by increasing the incident beam area accepted by the optic while keeping the optical aberrations low. Development of an optic which can improve the focused neutron beam intensity by increasing the incident beam area accepted by the optic while keeping the optical aberrations low represents a great improvement in the field of neutron optics and satisfies a long felt need of the optical engineer.